The insider told People: "Even after they decided to separate, they spent time together just the two of them. They always had fun and enjoyed hanging out. They never even dated other people after they separated."
Stiller moved back into the family home in 2020 so he could spend time with his children - Ella, 19, and Quinlin, 16, - amid the Covid-19 lockdowns.
Stiller recently admitted the situation later "evolved" between him and Taylor.
He said: "Then, over the course of time, it evolved. We were separated and got back together and we're happy about that.
"It's been really wonderful for all of us. Unexpected, and one of the things that came out of the pandemic."
The couple now have a "respect" for their similarities and differences.
He explained: "I think we have a respect for the ways that we're similar and the ways we're different.
"And I think accepting that, you can really appreciate someone more because you're not trying to get them to change for you.
"Once you accept that, you save a lot of energy. 'This is something that works for me; this is something that doesn't work for me.'
"If you have that trust level with your partner, you know that me saying 'I don't like doing that thing,' is not me saying 'I don't like you.'"
